The invention relates generally to insulated piping, and, more particularly, to prefabricated vacuum-insulated pipe sections and a method of joining them.
Thermally insulated pipes have a wide variety of industrial applications. For example, insulated piping is used to transport cryogenic liquids, that is, liquids having a boiling point at a temperature below -150.degree. F. at atmospheric pressure, between storage tanks or between a storage tank and a use device. Other examples include utilization in chemical plants or petroleum refining.
A major source of thermal leakage in insulated pipe systems is at the junction between two pipe sections. It is therefore preferable to eliminate such pipe joints. This would require, however, that the insulating piping be custom-manufactured to fulfill the length requirements of specific applications. Such an arrangement would be prohibitively expensive. As a result, efforts have been directed towards developing prefabricated insulated pipe sections that may be connected with joints that suffer from minimal thermal leakage. An appropriate number of such sections may be delivered and joined in the field to create efficient piping systems of desired lengths.
One type of prefabricated insulated pipe features sections constructed of copper pipe surrounded by foam insulation. The foam insulation is surrounded and protected by a PVC plastic casing. Such foam preinsulated pipe is offered under the names X-50 by Process and Cryogenic Services, Inc. of San Jose, Calif. and INSULTEK by Insultek Piping Systems of Manlius, N.Y.
Sections of foam preinsulated pipe are typically joined by brazing or butt-welding. Alternatively, the ends of the sections may be threaded. Foam insulation is next wrapped, placed, foamed or fitted around the joint. A "clam shell" is then placed over the foam insulation so as to protect it and secure it in place. The claim shell is typically constructed of stamped thin metal or molded plastic and features two halves that are riveted, glued or otherwise joined together when the clam shell is in position.
While the above foam preinsulated pipe system offers the advantage of low cost, higher performance levels are required by a number of present-day applications. The efficiency of the foam preinsulated pipe is typically 1435 BTU/hr overall heat loss per 100 feet of 1 inch pipe when liquid nitrogen is carried. In addition, greater service life is desired. Foam preinsulated pipe often needs extensive repair or replacement after a short service life to maintain insulating performance at original levels.
Another type of prefabricated pipe features vacuum-insulated sections. This system is offered by MVE, Inc. of New Prague, Minn. under the name VIP. A VIP pipe section is constructed of an inner pipe surrounded by an outer pipe. The inner pipe and outer pipe are concentrically positioned so that an annular insulation space is formed therebetween. The ends of the outer pipe are welded to the inner pipe so that the annular space is sealed. Either the inner pipe or the outer pipe is provided with a bellows in its middle portion so that the integrity of the welds is preserved when the pipes expand or contract by differing amounts due to temperature changes.
The insulation space of the VIP pipe is evacuated and filled with a multiplicity of layers of thin fabric formed of fine fibers of glass, cellulose or other fibrous material characterized by low heat conductivity. Positioned between the thin fabric layers are reflective barrier sheets formed of highly reflective material, such as a aluminum foil. When utilized in cryogenic liquid applications, the insulation arrangement of VIP pipe minimizes conduction and radiation heat gain to the cryogen within the inner pipes.
While VIP pipe sections may be connected in a manner similar to foam-insulated pipes, that is, with insulated clam shells, they are optimally joined by a bayonet connector arrangement such as is disclosed in U.S. Pat. No. 4,491,347 to Gustafson. The '347 patent discloses pipe sections that have interfitting metal male and female end portions. The material of the female portion has a coefficient of expansion that is greater than that of the male portion. As a result, when cryogenic liquid flows through the pipe sections, the female portion contracts into sealing relationship with the male portion.
Vacuum-insulated pipe such as the VIP is at the opposite end of the spectrum from foam preinsulated pipe in terms of both performance and cost. More specifically, the efficiency of the VIP system is much higher, with an overall heat loss of only 160 BTU/hr for 100 feet of 1 inch pipe when liquid nitrogen is carried. In addition, vacuum-insulated pipes such as the VIP typically retain their original insulating capabilities for fifteen to twenty years. The complexity of vacuum-insulated pipe with bayoneted ends, however, makes piping systems such as the VIP expensive to produce.
A further disadvantage associated with existing vacuum-insulated piping is that when the expansion/contraction bellows comprises a portion of the outer pipe, it is exposed to the weather and may otherwise be damaged. Alternatively, if the bellows comprises a portion of the inner pipe, it is difficult to repair and replace.
Accordingly, it is an object of the present invention to provide an insulated pipe that may be produced and installed in prefabricated sections.
It is another object of the present invention to provide an insulated pipe that offers high insulative efficiency at a reasonable cost.
It is another object of the present invention to provide an insulated pipe that has a long service life.
It is still another object of the present invention to provide an insulated pipe that is durable and that may be easily repaired.